1. Field of the Invention
Aspects of the present invention relates to an image forming apparatus, and more particularly, to a printing medium alignment device and an image forming apparatus having the same, which aligns a front end of a printing medium supplied to a printing unit.
2. Description of the Related Art
An image forming apparatus is an apparatus that prints a black and white image, or a color image, on a printing medium, e.g., paper, according to an image signal. An image forming apparatus can be, for example, a laser printer, an ink-jet printer, a copying machine, a multi-function printer, a fax machine, etc. An image forming apparatus can be classified as an electrophotographic type, in which a beam is scanned to a photosensitive body to form an electrostatic latent image and a developer is supplied to the electrostatic latent image to transfer the same onto a printing medium; or an ink-jet type, in which a liquid ink is ejected onto the surface of a printing medium, according to an image signal.
An image forming apparatus generally includes: a printing medium supply unit, which contains sheets of printing media, and supplies the printing medium sheet by sheet; a printing unit, which prints letters or pictures on the printing medium supplied from the printing medium supply unit; and a printing medium discharge unit, which discharges the printed printing medium. In the electrophotographic image forming apparatus, the printing unit includes: a photosensitive body, on which an electrostatic latent image is formed; an exposure unit that radiates light to the photosensitive body; a developing unit that supplies a developer to the electrostatic latent image formed on the photosensitive body, to form a visible image; a transfer unit that transfers the visible image to the printing medium; and a fusing unit that fuses the visible image to the printing medium, by applying heat and pressure to the printing medium. In the ink-jet image forming apparatus, the printing unit is provided with a print head that ejects an ink onto the surface of the printing medium, according to an image signal.
The both types of image forming apparatuses further include a printing medium alignment device, which is provided upstream from the printing unit (with respect to a paper transfer direction), to uniformly align the front end of the printing medium supplied from the printing medium supply unit. The printing medium supplied from the printing medium supply unit may get twisted/misaligned when it is fed to the printing unit. Such a misalignment may result in reduced printing quality. To prevent this problem, the printing medium alignment device aligns the printing medium, before the printing medium advances to the printing unit.
An example of a conventional printing medium alignment device is disclosed in Japanese Patent Laid-Open Publication No. 2004-26343 (published on Jan. 29, 2004). The disclosed printing medium alignment device includes: a register roller that rotates by receiving a driving force; an idle roller that contacts the register roller, and rotates together with the register roller; and a register shutter that is mounted so as to contact the front end of the printing medium, as the printing medium advances into a nip formed between the register roller and the idle roller. If the front end of the printing medium is properly aligned, the register shutter rotates to pass the printing medium.
The register shutter is provided with plural front-end touch parts, which contact the front end of the printing medium. When an external force, which is larger than a predetermined force, is applied, the register shutter is rotated. If the printing medium gets twisted, and thus, the front end of the printing medium does not contact all of the front-end touch parts, the pushing force is too weak to rotate the register shutter, so the register shutter continues to block the printing medium. If the printing medium is aligned, and thus, the front end of the printing medium contacts all the front-end touch parts, the register shutter rotates, and the printing medium progresses between the register roller and the idle roller.
For the register roller to align the front end of the printing medium, the front-end touch parts should be arranged in parallel, in a direction perpendicular to a feeding direction of the printing medium, and should not be bent nor deformed. In order to prevent wear and bending of the register shutter, the register shutter is typically made of a material mixed with a glass fiber.
However, because the register shutter successively contacts the printing medium, noise occurs when the printing medium pushes the register shutter and progresses. Further, the noise increases when the printing operation is performed at a high speed.